The aim of this research is to elucidate the controls governing the proliferation and differentiation of a multipotent stem cell, the interstitial cell (i-cell) of hydra. The approach is to alter the cell composition of the tissue and study the i-cell division and differentiation behavior. The cell composition can be altered by selective deletion of cell types in intact animals, by grafting pieces of the animal to one another, and with cell aggregation technique. In the last, cell suspensions of varying composition are made, aggregates formed which develop into animals in which i-cell behavior can be analyzed. The patterns of i-cell behavior will be analyzed quantitatively with methods developed for quantitating all cell populations of hydra. Quantitative kinetic studies will be carried out autoradiographically with isotopically-labelled cohorts of cells. The specific objects of this proposal are to (1) study the basis for position-dependent differentiation of interstitial cells; (2) study the capabilities of migrating interstitial cells; (3) study the complexity of the interstitial cell population; (4) develop a method for obtaining animals with a clonal interstitial cell population.